The present invention relates to a method and apparatus for applying waterborne coating compositions and to the resultant coated articles
Waterborne coating compositions have become increasingly important today due to organic volatile emissions regulations. Waterborne coating compositions are utilized to meet these regulations because the waterborne coating compositions contain much smaller amounts of organic volatiles than conventional solvent coating compositions
The application of waterborne coatings is complicated by the fact that the evaporation rate of water is dependent on the relative humidity of the spray environment. When the percent relative humidity is high (e.g., greater than 80%) water does not evaporate at a fast enough rate and "wet" paint films are produced. "Wet" films have a tendency to sag, pop, and in cases of metallic pigmented coatings produce dark, mottled films that exhibit areas of non-uniform distribution of the metallic pigment. Sags are unsightly, gravity driven flows that occur on vertical surfaces, and pops are small pore-like flaws in the coating that are caused by the violent evolution from the film of trapped solvent (either organic solvent or water). Conversely, when the percent relative humidity is low (e.g., less than 50%) water evaporates too quickly, and "dry" paint films are produced. "Dry" films tend to be rough and do not exhibit good leveling and flow characteristics.
There have been a number of attempts to overcome these problems, but none of them have been particularly successful. One approach has been to try to control the atmosphere in the spray booth or spray zone. U.S. Pat. Nos. 3,979,535 and 4,687,686 describe methods of controlling the temperature and humidity of the air in the spray booth to acceptable preset levels. If, however, the spray booth is not enclosed and is instead open to a work room or plant, it may be very difficult and expensive to control the temperature and humidity in the whole room or plant. Sometimes it is not feasible or desired to enclose the spray area.
U.S. Pat. No. 4,132,357 describes a method of applying solvent-thinned coatings utilizing a shroud which delivers air at a controlled temperature and humidity around the atomized spray, forming a controlled localized atmosphere. However, ambient air from the spray booth may mix with this controlled atmosphere as it passes from the spray gun to the article being coated, and the controlled atmosphere may dissipate very rapidly.
Another approach is to add water to the paint film to control evaporation of water from the film. U.S. Pat. No. 4,344,991 describes depositing a mist layer of water on top of or beneath the deposited waterborne coating by simultaneously atomizing water with ancillary nozzles. The water mist is not intended to intermix with the atomized waterborne coating. U.S. Pat. No. 4,396,651 describes a coating process in which water is atomized through nozzles ancillary to the main spray nozzle. The atomized water is said to build a uniform water atmosphere around the waterborne coating atomized stream without being homogeneously mixed with the coating. Both of these patents describe approaches that have problems with nozzle configuration complexities and have problems with accurately controlling the amount of water deposited or added to the constantly changing atmosphere around the atomized spray.
U.S. Pat. No. 4,341,821 describes spraying an organic solvent onto a coated article either before, after, or at the same time the waterborne coating is being applied. The solvent is intended to control the viscosity of the waterborne coating to prevent sagging. Spraying solvent defeats the main purpose of using waterborne coatings.
It would be desirable to overcome the problem of applying waterborne coatings compositions at varying relative humidities while avoiding the disadvantages of the prior art approaches. In particular, it would be desirable to avoid batch mixing of coatings with solvents, to avoid complicated, non-standard spray gun structures, and to avoid costly humidity control schemes.